Stoneware, earthenware, teracotta and porcelain - Almost the same, yet completely different

At first it sounds the same and also describes almost the same, but only almost. I would like to add clay, plaster, chalk, peat and clinker or bricks to the description. The materials are obviously similar, because they are still completely different materials with different properties and characteristics.

The material clay is generally a non-biological (= inorganic) mixture of various origins. The clay we use has developed over time from the weathering of rock, especially from feldspars. The weathering has gnawed away at entire mountain ranges and thus eroded even the hardest rock to gravel, sand or even clay. Clay particles are not larger than < 2 µm (= 0.000002 cm) and look like small scales or platelets under the microscope. This is also the reason why clay has to be kneaded before processing: The scales have to be aligned, otherwise tensions will occur within the work piece during drying.
Clay has accompanied mankind for more than 20,000 years as a basic material for building and artistic design. The most prominent example of the arts is the Venus of Dolní Věstonice - its age is estimated at 30,000 years!

Peat, on the other hand, is an organic material and should only be mentioned here briefly and for the sake of completeness. Peat is the result of the weathering of plants and other living material. It is a diffuse material mixture of a completely different origin than clay and is therefore also suitable for heating, for example - clay, on the other hand, can easily withstand temperatures of 1,300 °C and more and is not flammable. This is also one of the reasons why clay and ceramics are still used today as electrical insulators. Unlike plastic, clay is not flammable.

The classic white chalk is a very fine-grained, mostly white or light grey limestone and the basic material for writing chalk. Chalk consists essentially of calcium carbonate and is the remains of organisms from the Cretaceous period. In chalk, there are often numerous remains of larger organisms with a limestone skeleton, e.g. sponges, mussels and cephalopods. Large (former) decomposition products of this sought-after natural product can be found on the German Baltic Sea island of Rügen and the British white cliffs of Dover.

Loam, like clay, is a non-biological mixture. It consists of clay, silt and sand. Depending on the location and use, the loam is used fired to make clay bricks or unfired for plastering. Burned loam is more resistant to weathering than unburned loam, which can be easily liquefied again with water. Due to the lower firing temperature, however, fired loam is not as weather-resistant as clay. In the Orient, entire cities were built out of loam - due to the rare rainfall, it is an ideal building material. The city of Aït-Ben-Haddou in Morocco is almost 1000 years old and built almost entirely of loam.

Gypsum is also a non-biological material, but it is a pure material. Gypsum is the very common mineral gypsum spar (= calcium sulphate). In general, gypsum is colourless or white, but it can take on a yellowish, reddish, grey or brown colour due to the various admixtures. Pure gypsum that has been subjected to thermological processes is also known under the cultural term alabaster. The transparent Marienglas is, like the well-known sand roses, another variety of plaster.

The earthenware clay is mainly used in the rotary pottery. Due to its greasiness and dynamic malleability it is easy to work on the lathe. Likewise, this lathe clay is free of chamotte. Chamotte is clay that has already been fired and ground once and is added to the raw mass. The undermixing makes the ceramic more stable, which is an important aspect especially for build-up ceramics. The increased stability of the ceramic allows for larger objects. A common proportion of chamotte in the build-up ceramic is 25-40% with a grain size of up to 0.5 mm. The higher the proportion of fireclay, the more suitable the clay is for abutment work. Large abutments such as 1:1 nude plastics work with a fireclay content of > 40% and a grain size around 1 mm. The earthenware clay is available in a wide range of colours and fireclay.

The firing temperature of earthenware clay is between 1,000 - 1,150 °C. These relatively low temperatures are not sufficient to allow the silicate contained in the clay to crystallize completely. After firing, the clay remains predominantly porous and is therefore not 100% dense. For utility ceramics, the tightness is of course also of interest. After a short time, the ceramics are sintered by the suspended matter contained in the tea or water and after a while of use they become absolutely watertight. To achieve absolute tightness, it must be treated with a glaze.
Terracotta ceramics is a popular form of earthenware clay. Since the mostly red ceramics are not waterproof, water can penetrate into the open pores in winter and break them when freezing. In the Mediterranean region, where this ceramic originates, this is not a problem due to the mild winters. In our latitudes, the only thing that helps in winter is to cover them with insulating material or, best of all, to bring them in.

The stoneware clay is also to be obtained and used with or without fireclay. However, the clay is less greasy and therefore less malleable or flexible. You will not notice this in the build-up ceramics, but also in the potter's wheel. Due to the increased proportion of silicate crystals (in principle glass), strongly bulbous vessels can only be produced with difficulty. The mass is less pliable and is less easy to form. But this increased proportion of crystals allows the clay to withstand much higher temperatures without losing its shape or boiling. Stoneware clay is usually fired at between 1,100 and 1,300 °C. At temperatures above 1150 °C the silicate contained in the clay melts out and seals the ceramics from the inside. Stoneware ceramics do not necessarily require a glaze. Another interesting property is the sound of these ceramics - it is very close to bronze bells and resonates for a long time.

The iron-free porcelain clay is the purest clay found in nature. In nature, these pure clay deposits occur only very rarely. This white substance, as its name suggests, is used, among other things, to make porcelain. When fired, this type of clay becomes very hard and glassy. Due to the low proportion of foreign minerals and high quartz content, this material can be fired at a very high firing rate. Temperatures of 1500 °C are common for porcelain and lead to the desired translucent character. Unlike classical clay, porcelain is cast and not shaped. In plaster moulds, 2-3 mm thin cups can be produced without any problems - only these are translucent and explain the historical greed for this material. In theory, porcelain can be processed at the turntable, but it is very laborious and does not produce such good results by far. Due to the high quartz content, the mass is much less malleable than clay and its consistency is more reminiscent of chewing gum.

The first processing of this rarity is dated around the turn of time and was from then on further optimized. From the 15th century onwards, porcelain ceramics, primarily tableware, were imported to Europe on a large scale by the Spanish and Portuguese. This was necessary because the Chinese potters kept the recipe secret for a long time.
In January 1708 Johann Friedrich Böttger and Ehrenfried Walther von Tschirnhaus in Dresden succeeded in producing the first European hard porcelain - a breakthrough. From this rediscovery onwards, competitive porcelain was produced in Germany due to the rich kaolin deposits in northern Bavaria, Upper Franconia, the Upper Palatinate and Thuringia. A short time later, manufactories were also founded in other parts of Europe and porcelain was made accessible to a wide range of people.

Clinker and brick has been a popular building material in Germany for about 500 years and is one of the harder building materials. Entire districts of Berlin and other cities were regularly built with clinker bricks during the industrial revolution. It easily withstands heat and frost, is inexpensive and is also a good heat insulator. As a rule, pure clay is not used in its manufacture. It has the property that it shrinks considerably during drying and becomes extremely hard during the burning process. This usually makes it harder than it should be - a certain elasticity is an advantage. For brick products the clay is therefore usually mixed with various additives, which give it the properties desired by the manufacturers.